Sorry, but I don't believe that you can. Preproduction BIOSs are not made available via the Intel Download Center. BIOS 34 (HNKBLi70.86a.0034) is the only production BIOS release that is available. You can obtain it here: https://downloadcenter.intel.com/download/27724/NUCs-BIOS-Update-HNKBLi70-86A-?product=126141 Download BIOS Update [HNKBLi70.86A].
Perhaps if you explain your need for this preproduction BIOS, we can see if this request can be fulfilled (offline).
Thank you for your response and your patience.
Please go ahead and test the latest BIOS release back in 5/16/2018 and let us know the outcome:
I really hope to hear from you soon.